The present invention relates to transformers and more particularly to encapsulated transformers that are completely watertight.
Transformers can be used for stepping up alternating current to high voltages for long-distance power transmission in order to minimize the relative voltage losses and also for stepping down the voltage at the point of consumption. In some applications, such as, for example, in air field runway lighting systems which aid aircraft pilots at night or during periods of poor visibility, it is necessary that the transformer be enclosed in a permanently sealed case so as to be completely watertight. The transformer should also be capable of continuous outdoor service at a wide range of temperatures (e.g., from xe2x88x9255xc2x0 C. to +65xc2x0 C.), and should operate properly when submerged in the ground with up to 5,000 volts on the primary windings. In addition, various applications also require that the encapsulated transformer exhibit other characteristics such as the ability to absorb shock without adversely affecting the operation of the transformer, the absence of internal air pockets insofar as practicable, and the ability to withstand exposure to sun, oil, gasoline, moisture, acid and alkaline soils.
In the past, transformers have been encapsulated in rubber in order to achieve the necessary watertight characteristics. The manufacture of a rubber encapsulated transformer typically involves the placement of the transformer coil assembly in a mold into which melted rubber is fed. In such processes, it has been known to use a flexible vulcanized rubber lead aligning base member 20 such as that illustrated in FIGS. 1 and 2. Such a lead aligning base member 20 includes a bottom portion 22 and an upwardly directed portion 24. The bottom portion 22 is provided with grooves 26 which serve the purpose of aligning the primary and secondary leads extending from the transformer coil assembly.
The manufacture of an encapsulated transformer utilizing a flexible vulcanized rubber lead aligning base member 20 such as illustrated in FIGS. 1 and 2 typically involves the following steps. The primary and secondary leads extending from the transformer coil assembly are properly aligned and the flexible vulcanized rubber lead aligning base member 20 is assembled into the transformer coil assembly so that the upwardly directed portion 24 of the base member 24 extends through the opening in the core. The grooves 26 on the lead aligning base member 20 ensure that the leads are properly positioned and captivated. The transformer coil assembly with the flexible vulcanized rubber lead aligning base member 20 is then positioned in the bottom of a mold over an upward protruding pin in the mold. Prior to placing the coil assembly and the flexible base member into the mold, one or more layers of unvulcanized rubber material is placed in the bottom of the mold in order to allow identification indicia to be stamped into the outer surface of the encapsulated transformer once the molding operation is complete. Once the transformer coil assembly with the flexible lead aligning base member 20 is positioned in the mold, the mold is closed and rubber is forced to flow into the mold at a relatively low injection pressure until the mold is filled. The filled mold, which is heated, is then held under pressure for a period of about 45 minutes in order to properly cure the rubber. Once the curing is complete, the mold is opened and the encapsulated transformer is removed.
The production of rubber encapsulated transformers is susceptible of certain improvements. For example, the amount of time required to fabricate a single rubber encapsulated transformer (i.e., approximately 45 minutes) is not readily conducive to high output with a relatively few number of molds. Indeed, the encapsulation of transformer coils assemblies in rubber typically requires numerous molds in order to meet the desired production requirements for a given facility.
Attempts have apparently also been made to encapsulate transformer coil assemblies in thermoplastic rubber. However, it has been found that such attempts have been limited to relatively small size transformers. That is believed to be due, at least in part, to the fact that a larger transformer coil assembly requires greater amounts of thermoplastic rubber to completely encapsulate the transformer coil assembly. Since the thermoplastic rubber must be injected into the mold in a very short period of time in order to prevent the thermoplastic rubber from curing prior to complete encapsulation of the transformer assembly, significant pressures are developed within the mold during the injection process. It is believed that the significant pressures developed during encapsulation of larger transformer coil assemblies were so great that it was deemed practically impossible to produce a defect-free, completely watertight transformer encapsulated in thermoplastic rubber.
In light of the foregoing, it would be quite desirable to be able to produce an encapsulated transformer that is completely watertight and impervious to air. Moreover, it would be desirable to produce such an encapsulated transformer in a relatively short period of time to facilitate high quantity production.
In accordance with one aspect of the present invention, an encapsulated transformer comprises a transformer coil assembly that includes a core having a hollow interior, primary wire windings and secondary wire windings, at least one primary lead connected to the primary windings, at least one secondary lead connected to the secondary windings, and a rigid core insert having an upstanding portion positioned within the hollow interior of the core and a base portion on which the core rests. A unitary thermoplastic rubber jacket completely surrounds the transformer coil assembly and is bonded to the base portion of the core insert in order to provide a completely waterproof encapsulated transformer.
In accordance with another aspect of the present invention, a method for producing an encapsulated transformer comprises providing a transformer coil assembly that includes a core having a hollow interior, primary wire windings and secondary wire windings, placing a rigid core insert centrally in a mold cavity of a mold, and positioning the transformer coil assembly in the mold cavity so that the transformer coil assembly rests on a base portion of the core insert while an upstanding portion of the core insert is positioned in the hollow interior of the core. The mold cavity is then closed and thermoplastic rubber is injected into the mold cavity to completely encapsulate the transformer coil assembly in a jacket of thermoplastic rubber and bond the thermoplastic rubber to the base portion of the core insert. After a relatively short period of time, the mold cavity is opened and the encapsulated transformer coil assembly with the jacket of thermoplastic rubber is ejected from the mold cavity, thereby resulting in a completely waterproof encapsulated transformer.
In accordance with another aspect of the present invention, a method of producing a completely encapsulated watertight transformer comprises providing a mold which has a mold cavity and at least one dowel pin positioned in a bottom of the mold cavity. The dowel pin is disposed in a hole in the mold so that a vent space which communicates with the mole cavity exists between an outer periphery of the dowel pin and an inner periphery of the hole. The method also involves providing a transformer coil assembly that includes a core having a hollow interior, primary wire windings and secondary wire windings wound around the core, and providing a core insert that includes a base portion, an upstanding portion extending from the base portion and a hole extending through the base portion and into the upstanding portion. The core insert is placed within the mold cavity so that the base portion rests on the bottom of the mold cavity while the vent space defined by the dowel remains uncovered. After the mold cavity is closed, thermoplastic rubber is injected into the mold cavity. During the injection of the thermoplastic rubber, gas within the mold cavity is vented by allowing gas to escape to the exterior of the mold through the vent space. The mold cavity is opened after the transformer coil is completely encapsulated within a jacket of thermoplastic rubber, and the encapsulated transformer coil assembly is then ejected from the mold cavity to result in the completely watertight encapsulated transformer.
In accordance with a further aspect of the present invention, a mold for completely encapsulating a transformer coil assembly in a casing of thermoplastic rubber includes a bottom mold portion having a mold cavity formed therein that is sized and configured to receive a transformer coil assembly so that a space exists between an outer periphery of the transformer coil assembly and an inner surface of the mold cavity. The mold is provided with gates for allowing thermoplastic rubber to be injected into the mold cavity, and an upstanding pin that extends upwardly within the mold cavity from a bottom surface of the mold cavity for receiving a core insert. A movable mold cover is provided for opening and closing the mold cavity, and vent means are positioned in the bottom surface of the mold cavity for venting gas within the mold cavity to the exterior of the mold during injection of the thermoplastic rubber into the mold cavity. The vent means is positioned radially outwardly of the upstanding pin and communicates the mold cavity with the exterior of the mold.